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1 Structural Parameters in Coma Legacy Survey Leiden Astro-WISE meeting 2008 April 1 Marc Balcells (IAC)

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Presentation on theme: "1 Structural Parameters in Coma Legacy Survey Leiden Astro-WISE meeting 2008 April 1 Marc Balcells (IAC)"— Presentation transcript:

1 1 Structural Parameters in Coma Legacy Survey Leiden Astro-WISE meeting 2008 April 1 Marc Balcells (IAC)

2 2 Topics 1.Coma ACS structural analysis plan 2.Results to date 1.Sextractor photometric errors: realistic estimates 2.Structural parameter errors 1.Comparison GALFIT vs GIM2D Usefulness of Astro-WISE

3 3 Coma Structural Analysis Working Group (SAWG) Marc Balcells (Chair) IAC Organization. Galaxy synthetic models Rafael Guzmán UFL GIM2D Carlos Hoyos UFL/UAM GIM2D Reynier Peletier Groningen GALFIT Gijs Verdoes Kleijn Groningen GALFIT Harry Ferguson STScI Insert models into images Derek Hammer Hopkins Catalogs

4 4 SAWG mission Provide photometry and structural parameters of given catalogs Input: catalogs provided by Catalogs Team SAWG contribution to catalog generation: 1.Subtracting bright galaxies 2.Detection efficiency. Spurious sources. Photometric errors. Output: catalog of photometry and structural parameters

5 5 Increasing levels of structural analysis… 1.Mag, Color 2.Mag, Color, Elipticity, PosAng, Reff Sersic vs curve-of-growth 3. add Isophotal profiles (eg GALPHOT) 4. add Concentration-Asymmetry (CAS; GINI etc) 5. add Sérsic model:  e, R e, n 6. add B/D; Sersic Parameters; Disk Parameters Sersic+expon model 1D vs 2D GALFIT vs GIM2D 7. add nuclear components 8. add bars 9. add lopsidedness

6 6 Three stages Balcells & Peletier 2007 “The Structural Analysis of the Coma ACS Legacy Images” –Three Phases Phase 1: SExtractor Phase 2: GALPHOT isophotal analysis Phase 3: 2D models (GALFIT, or GIM2D), fixed centers 1.Pure Sersic: I, B, Re, nSer 2.Sersic+Expon: Ie, Re, nSer, mu0, h 3.Sersic+Expon+NuclearComp: Ie, Re, nSer, mu0, h, Inuc, Bnuc Public catalog –Coma Paper II, The Catalog (Derek Hammer et al. 2008) SExtractor-based MAG_AUTO (I, B), Flux radius, elipticity, pos angle Realistic errors from simulations of injecting synthetic sources into ACS images. Out of scope –Asymmetries; bars; truncations; anti-truncations; dust; color gradient; companions

7 7 SExtractor catalog errors SExtractor errors : two problem areas 1.Poisson errors based on background noise, underestimated when noise correlated 1.Charge transfer efficiency 2.Reduction: rebinning, convolving 2.Some flux always missing ~0.1 mag Simulations to address both problems

8 8 Synthetic image experiments Multi-dimensional problem –Mag, R eff, n Ser, eps Models randomly sampling this space About 300,000 models per band Techniques –Models by GALFIT –SExtractor run, destroy original model –CONDOR distributed software, ~180 linux workstations at IAC –Expensive, convolution with ACS psf.

9 9 Wings of stellar PSFs: King (1971)

10 10 Missing flux - PSF convolution PSF extended wings About 0.05 mag May be added as an aperture correction Does not show up in simulations if model PSF is truncated to ~4-5 FWHM

11 11 Missing flux - SExtractor truncation Often argued that sky overestimated, subtracts too much light from galaxy –Can happen, can be cured The basic SExtractor problem is truncation eps 0.4 n Ser > 2.5 n Ser < 2.5

12 12 SExtractor cuts at 2.5 R 1

13 13 Solution: compute flux outside 2.5 R 1 eps 0.4 n Ser > 2.5 n Ser < 2.5 eps 0.4 n Ser > 2.5 n Ser < 2.5 Offsets disappear errors at faint mu are symmetric

14 14 SExtractor errors after aperture corrections eps 0.4 n Ser 2.5

15 15 Region of interest in mag-Re diagram: Detection efficiency mag vs Re diagram

16 16 Choosing a code for 2D structural modeling Two codes optimized for automatic fitting –GIM2D (Simard et al 2003) –GALFIT (Peng et al. 2002) A recent comparison –Haussler et al 2007 (astro-ph/ ) GEMS team Conclude: –both codes deemed good –Devil is in the details - devil is in the sky! –Issue with companions / masking nearby objects / fitting simultaneously Us: our own tests. First step has been with exactly same models as in GEMS paper.

17 17 Experiments with GEMS models Two images from GEMS –Disk0001 (expon profiles) –Bulge0001 (deV profiles) Sextractor (Hoyos) GALFIT (Verdoes, Peletier) GIM2D (Hoyos, Guzman)

18 18 Our conclusions We reproduce conclusions of Haussler et al (2007) GIM2D can be better than reported by Haussler et al. at the expense of more manual intervention –But GIM2D is an automatic code GALFIT advantage is that it can fit more than two components –Sersic, Expon, Nuclear source

19 19 Astro-WISE Used by Groningen team Could other teams have done their simulations using Astro-WISE?? –Eg Carlos Hoyos, from Madrid, fitting Gim2D –Me: provide IRAF scripts to generate 1000’s bulge-disk models into Coma ACS images in astro-WISE

20 20 Is use of Astro-WISE desirable … … for entire Coma-ACS team? YES… Pros –Making processes more systematic, –Pre-plan steps –Quality control –History, memory of previous steps Difficulties –Find your way especially as you come into the system Wishes –Be able to operate on the data stored in Astro-WISE with or own codes

21 21 Astro-WISE for newcomers… … like me and most in the Coma Survey People coming from outside: –Want to get their thing done Without having to read (much) documentation The all-familiar IRAF case… You can flat-field, copy and display an image the first day. You only need a very skeletal knowledge to start: – tasks – epar task

22 22 CONclusions Clearly a very very powerful system Think more on user interface

23 23 Astro-WISE Astro-WISE might benefit from taking care of this level: the skeletal level of knowledge that allows the novice user to get something done –Once we know how to get something done, we will progressively learn the inner workings. Another example: look at my laptop –Underneath the smooth performance, lots of C++, classes, dictionaries –The user needs not know ANY of that. Mac OSX, a model of user interface –The user only thinks his own language –Apple, a long tradition of intuitive User Interface

24 24 Examples I=22 B/T = 0.2 i = 70 I=22 B/T=0.5 i = 70

25 25 Injection in ACS images


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